Symmetric transmit opportunity (TXOP) truncation

ABSTRACT

Various embodiments of symmetric transmit opportunity (TXOP) truncation (STT) systems and methods are disclosed. One method embodiment, among others, comprises receiving a frame that truncates a TXOP around a first station, and responsive to receiving the frame, sending a second frame that truncates the TXOP around a second station. Others system and method embodiments are disclosed.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a Continuation of U.S. application Ser. No.13/251,772, filed Oct. 3, 2011, which is a Divisional of U.S.application Ser. No. 11/651,879, filed Jan. 10, 2007 (now U.S. Pat. No.8,031,661), which claims the benefit of Provisional application U.S.Application 60/757,827, filed Jan. 10, 2006. U.S. application Ser. No.11/651,879 is also a Continuation-In-Part of U.S. application Ser. No.11/557,516, filed Nov. 8, 2006, which claims the benefit of Provisionalapplication U.S. Application 60/735,024, filed Nov. 8, 2005, andProvisional application U.S. Application 60/758,595, filed Jan. 11,2006, all of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present disclosure is generally related to communication systems andmethods and, more particularly, is related to collision avoidancesystems and methods in wireless networks.

2. Related Art

Communication networks come in a variety of forms. Notable networksinclude wireline and wireless. Wireline networks include local areanetworks (LANs), DSL networks, and cable networks, among others.Wireless networks include cellular telephone networks, classic landmobile radio networks and satellite transmission networks, among others.These wireless networks are typically characterized as wide areanetworks. More recently, wireless local area networks and wireless homenetworks have been proposed, and standards, such as Bluetooth® and IEEE802.11, have been introduced to govern the development of wirelessequipment for such localized networks.

A wireless local area network (LAN) typically uses infrared (IR) orradio frequency (RF) communication channels to communicate betweenportable or mobile computer terminals and access points (APs) or basestations. These APs are, in turn, connected by a wired or wirelesscommunications channel to a network infrastructure which connects groupsof access points together to form the LAN, including, optionally, one ormore host computer systems.

Wireless protocols such as Bluetooth® and IEEE 802.11 support thelogical interconnections of such portable roaming terminals having avariety of types of communication capabilities to host computers. Thelogical interconnections are based upon an infrastructure in which atleast some of the terminals are capable of communicating with at leasttwo of the APs when located within a predetermined range, each terminalbeing normally associated, and in communication, with a single one ofthe access points. Based on the overall spatial layout, response time,and loading requirements of the network, different networking schemesand communication protocols have been designed so as to most efficientlyregulate the communications.

IEEE Standard 802.11 (“802.11”) is set out in “Wireless LAN MediumAccess Control (MAC) and Physical Layer (PHY) Specifications” and isavailable from the IEEE Standards Department, Piscataway, N.J. The IEEE802.11 standard permits either IR or RF communications at 1 Mbps, 2 Mbpsand higher data rates, a medium access technique similar to carriersense multiple access/collision avoidance (CSMA/CA), a power-save modefor battery-operated mobile stations, seamless roaming in a fullcellular network, high throughput operation, diverse antenna systemsdesigned to eliminate “dead spots,” and an easy interface to existingnetwork infrastructures. The IEEE Standard 802.11b extension supportsdata rates up to 11 Mbps.

The current 802.11 standard describes several methods to set a virtualcarrier sense, referred to as a network allocation vector or NAV, mostnotably by request to send/clear to send (RTS/CTS) mechanisms. Thesending of an RTS frame (herein, RTS frame also referred to as, simply,RTS) sets a NAV locally around the sender of the RTS, and the sending ofa CTS frame (herein, CTS frame is also referred to as CTS) does the samelocally around the sender of the CTS (e.g., the receiver of the RTS).

One problem that exists in current implementations under 802.11 involveswhat is referred to as a hidden node problem. For instance, in aninfrastructure mode of a wireless LAN system, a first device maycommunicate frames to the AP and vice versa. Similarly, a second devicemay communicate frames to the AP, and vice versa. However, the seconddevice may not detect transmissions from the first device (hence thephrase hidden node), for instance if the distance between the first andsecond device is too great. Because of the hidden node problem, variouscomplications may arise in terms of collision avoidance and symmetry ofresponse among devices, causing inequity in terms of opportunities foraccess to a shared medium of the communication system. For instance, in802.11 compliant systems, the NAV can be reset by an access point (AP)through the transmission of what is commonly referred to as a CF-endframe. One exemplary frame format for a CF-end frame 10 is shown in FIG.1, and comprises two octets (exemplary quantity of octets shown beneaththe frame 10) corresponding to a frame control field 102, two octetscorresponding to duration field 104, six octets corresponding to areceiver address (RA) field 106, which usually contains the BroadcastAddress (BC), six octets corresponding to a basic service set identifier(BSSID) field 108, and four octets corresponding to a frame checksequence (FCS) field 110. When the distance between a client station andan AP is large, one possible effect is that the area where the CF-endframe 10 is not received will not reset the NAV, resulting ininequitable access as described above.

SUMMARY OF THE INVENTION

Embodiments of the present disclosure provide symmetric transmitopportunity (TXOP) truncation systems and methods in a wireless network.Briefly described, one embodiment of a method, among others, comprisesreceiving a frame that truncates a TXOP around a first station, andresponsive to receiving the frame, sending a second frame that truncatesthe TXOP around a second station.

Another method embodiment, among others, comprises receiving a framefrom an access point (AP) that includes an indicator of an end of aTXOP, responsive to receiving the indicator, sending a first frame thattruncates the TXOP around a first station, receiving the first frame atthe AP, and responsive to receiving the first frame at the AP, sending asecond frame that truncates the TXOP around the AP.

Another method embodiment, among others, comprises sending at a firststation a first frame, sending at the first station an extendedinterframe space (EIFS) set frame after the first frame, and sending ata second station a second frame having a completed transmissioncorresponding to time based on an interval of EIFS less DIFS, theinterval commencing from an end of the short frame.

One system embodiment, among others, comprises a processor configuredwith logic to receive a frame that truncates a TXOP around a firststation and, responsive to receiving the frame, the processor furtherconfigured with the logic to send a second frame that truncates the TXOParound a second station.

Another system embodiment, among others, comprises means for receiving aframe that truncates a TXOP around a first station, and responsive toreceiving the frame, means for sending a second frame that truncates theTXOP around a second station.

Other systems, methods, features, and advantages of the presentdisclosure will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a block diagram that illustrates one exemplary frame formatfor a CF-end frame.

FIG. 2 is a block diagram of an exemplary environment in which variousembodiments of a symmetric transmit opportunity (TXOP) truncation (STT)system may be implemented.

FIGS. 3A-3B are block diagrams of that illustrate extended interframespace (EIFS) avoidance aspects to embodiments of the STT system as shownin FIG. 2.

FIG. 4 is a block diagram that illustrates a mechanism employed by oneembodiment of the STT system shown in FIG. 2 for an access point (AP)initiated TXOP where the AP signals the end of TXOP by setting an end ofTXOP (EOT) indication on the final transmission to a client.

FIG. 5 is a block diagram that illustrates an alternative to themechanism shown in FIG. 4 where the AP sends a CF-end with a clientaddress, which is followed by a CF-end from the client with the sameaddress of the client.

FIG. 6 is a block diagram that illustrates an alternative to themechanism shown in FIG. 4 where the AP sends a CF-end with the clientaddress in the location of a basic service set identifier (BSSID) fieldand the client responds with a CF-end with the BSSID and the AP respondswith a final CF-end.

FIG. 7 is a block diagram that illustrates a mechanism employed by oneembodiment of the STT system shown in FIG. 2 for a station-initiatedTXOP that is terminated by a first station sending a CF-end with theaddress of the second station in the location of the BSSID field and thesecond station responding with a CF-end with the second station addressin the BSSID field.

FIG. 8 is a block diagram that illustrates a mechanism employed by oneembodiment of the STT system shown in FIG. 2 with a BSSID of a stationwhich either transmitted the latest beacon in an independent basicservice set (IBSS) or an AP in a Direct Link communication.

FIG. 9 is a block diagram that illustrates a mechanism employed by oneembodiment of the STT system shown in FIG. 2 for a station-initiatedTXOP that is terminated by a first station sending a CF-end to a secondstation, which responds with a CF-end to the third station, the latterwhich responds with a final CF-end.

FIG. 10 is a flow diagram that illustrates a method embodimentcorresponding to the mechanism shown in FIG. 4.

FIG. 11 is a flow diagram that illustrates a method embodimentcorresponding to the mechanisms shown in FIGS. 5 and 7.

FIG. 12 is a flow diagram that illustrates a method embodimentcorresponding to the mechanisms shown in FIG. 6.

FIG. 13 is a flow diagram that illustrates a method embodimentcorresponding to the mechanisms shown in FIGS. 8-9.

FIG. 14 is a block diagram that illustrates a NAV reset mechanismemployed by an embodiment of the STT system shown in FIG. 2.

FIG. 15 is a block diagram that illustrates an alternative to the NAVreset mechanism shown in FIG. 14 without using CF-end frames.

FIG. 16 provides a flow diagram that illustrates a method embodimentcorresponding to the NAV reset mechanisms shown in FIGS. 14-15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Disclosed herein are various embodiments of symmetric transmitopportunity (TXOP) truncation (STT) systems and corresponding mechanismsor methods in a wireless network (herein, individually or collectivelyreferred to also as an STT system or STT systems). The STT systemsdescribed herein comprise functionality to truncate a TXOP in asymmetric manner in a plurality of devices. That is, in various STTsystems and methods described herein, both access points (APs) andclient stations (herein, clients) are also allowed to truncate theirTXOPs by transmitting a CF-end frame. This truncation is referred to asa TXOP truncation, where a NAV is first set for some maximum duration oftime (e.g., the TXOP limit) and then reset when no more traffic (e.g.,of frames) is left and there is some NAV time that still remains. Ingeneral, a TXOP comprises a signal exchange which may begin with thetransmission of a short frame such as an RTS or CTS and ends with thelast frame sent or received by the sender of the short frame (e.g.,sender of the RTS or CTS). The truncation at both clients and APsresolve the various problems resulting from a reset that is notsymmetrical, such as in conventional systems where the reset comes froma single device (though set by both devices through the RTS/CTSmechanisms). Various embodiments of the STT systems provide one or moremechanisms for the symmetrical truncation of TXOPs.

Thus, although described mostly in the context of a wireless local areanetwork (WLAN) environment having a basic service set (BSS) configuredin an infrastructure mode, the various embodiments of the STT systemsdescribed herein may similarly be applied to other systems andenvironments, such as ad hoc systems (e.g., independent BSS (IBSS)systems) or DLC systems, as well as other communication systemenvironments. Additionally, although IEEE 802.11 is prominently usedherein as an example of a standard used in the disclosed exemplarywireless networks, the various systems and methods described herein mayapply to virtually any wireless network. Further, some embodiments ofthe STT systems may include some or all of the functionality ofcollision avoidance systems (CA systems) described in the co-pendingapplication having Ser. No. 11/557,516 and incorporated herein byreference in its entirety.

FIG. 2 shows an exemplary wireless LAN (WLAN) environment 100 in whichvarious embodiments of a symmetric transmit opportunity (TXOP)truncation (STT) system 200 may be implemented. In general, the STTsystem 200 is configured as a basic service set (BSS), which comprises aplurality of stations or nodes, such as stations 202, 204, and 206. Eachof the stations 202, 204, and 206 may be embodied as one of manywireless communication devices, including computers (desktop, portable,laptop, etc.), consumer electronic devices (e.g., multi-media players),compatible telecommunication devices, personal digital assistants(PDAs), or any other type of network devices such as printers, faxmachines, scanners, hubs, switches, routers, set-top boxes, televisionswith communication capability, etc.

The STT system 200 shown in FIG. 2 comprises, in one embodiment, anaccess point (AP) station 202 (herein, also referred to, simply, as anAP) and one or more client stations 204, 206 (herein, also referred toindividually or collectively as client(s)). The STT system 200 isconfigured in what may be referred to as an infrastructure mode, wherebyclients 204 and 206 communicate frames directly with the AP 202 and notwith each other. However, the various embodiments of STT systems 200 arenot limited to such arrangements, and in some embodiments, may bearranged in ad hoc or direct link communication configurations. Includedwithin each of the AP 202 and clients 204, 206 is control logic 300. Thecontrol logic 300 implements MAC layer and PHY layer services. The MAClayer services provide the capability for a given station to exchangeMAC frames. The MAC layer services provide management, control, or dataframes for transmission between stations 202, 204, and 206. The 802.11MAC layer services make use of at least three frame types (data,management, control), and within each type, various subtypes. Forinstance, request-to-send (RTS) frames, clear-to-send (CTS) frames, andCF-end frames are examples of control subtypes (e.g., control/CF-endrefers to a control-type frame of the subtype, CF-end). After a stationforms the applicable MAC frames, the frame bits are passed to the PHYlayer for transmission. One skilled in the art can understand in thecontext of this disclosure that the control logic 300 can be configuredusing a plurality of modules (e.g., hardware and/or software), each withdistinct functionality, or as a single module comprising a plurality offunctionality.

The control logic 300 can be implemented in hardware, software, or acombination thereof. When implemented in whole or in part by software,control logic 300 is implemented in software stored in a memory that isexecuted by a suitable instruction execution system. When implemented inwhole or in part by hardware, the control logic 300 can be implementedwith any or a combination of the following technologies, which are allwell known in the art: a discrete logic circuit(s) having logic gatesfor implementing logic functions upon data signals, an applicationspecific integrated circuit (ASIC) having appropriate combinationallogic gates, a programmable gate array(s) (PGA), a field programmablegate array (FPGA), etc. In one embodiment, the control logic 300 mayinclude a PHY layer processor, MAC layer processor, or a combination ofboth (in the same or separate units), including, but not limited to, adigital signal processor (DSP), a microprocessor unit (MCU), a generalpurpose processor, and an application specific integrated circuit(ASIC), among others.

The AP 202 is typically connected to a wired network (e.g., Ethernet),not shown. In general, the clients, such as client 204, connect to theAP 202 through a scanning process. The scanning process can either beperformed passively by listening for a beacon transmitted by one or moreAPs 202, or actively by sending out probes to one or more APs 202 andchoosing an AP that provides the best connection (e.g., in terms ofsignal strength and/or bit error ratio (BER)). After an AP is chosen,such as AP 202, an authentication process occurs between the client 204and the AP 202, and then association between the client 204 and the AP202 may begin.

Association involves the communication between the clients 204, 206 andthe AP 202 via a shared wireless medium 208. In one implementation, theclient 206 may represent a hidden node to client 204, and vice versa.Various mechanisms or methods for signal exchanges are illustrated inFIGS. 3A-16 to highlight various features of embodiments of the STTsystem 200. Although described below in the context of the AP 202 orclient 204 (or similarly client 206) sending or receiving varioustransmissions, it can be understood in the context of this disclosurethat the effectuating of the various functionality of the STT system 200is through the control logic 300 of each station (node). Further, thevarious interframe spaces (e.g., extended interframe space (EIFS), shortinterframe space (SIFS), etc.) described in 802.11 and understood bythose having ordinary skill in the art are omitted from the variousfigures (and corresponding description) except where helpful to theunderstanding of the various embodiments.

Before describing the various symmetrical TXOP truncation mechanisms ofembodiments of the STT system 200, an embodiment of an STT system 200that performs an AP-initiated TXOP is described in association withFIGS. 3A-3B. That is, FIGS. 3A-3B show one mechanism used by anembodiment of the STT system 200 to set a NAV and/or avoid EIFS for anAP-initiated TXOP. Referring to FIG. 3A, the AP 202 sets a NAV prior toa TXOP 302 by starting the TXOP 302 with a short frame 304 (e.g., shortto reduce the probability of error) at a low basic rate. Note thatblocks 302 and 304 collectively comprise a TXOP. The short frame 304contains a duration value (e.g., the NAV setting) equal to the expectedlength of the TXOP 302. Referring to FIG. 3B, the AP 202 avoids any EIFSat the end of the TXOP 302 by sending a short frame 306 (typicallycontaining a duration value of zero) at a low basic rate as the finalframe of the TXOP 302. Note that blocks 302 and 306 collectivelycomprise a TXOP. In each case, the rate is selected such that framesfrom the AP 202 reach all nodes in the network (e.g., BSS).

The description that follows and the associated figures (FIGS. 4-9)disclose various mechanisms employed (implemented) by certainembodiments of the STT system 200 that provide for symmetrical TXOPtruncation using CF-end frames (herein, each of these frames also simplyreferred to as CF-end). In particular, various mechanisms ofcommunication between devices of one or more STT system embodiments areillustrated in FIGS. 4-9 and explained in the corresponding text. Forinstance, shown in FIG. 4 are blocks representing respective frames thatare passed between the client 204 and the AP 202 to provide forsymmetrical TXOP truncation. As noted from FIGS. 4-9, the AP-sourcedframes are represented with blocks in the top row (indicated inparenthesis, with “AP”), and the client-sourced frames are representedwith blocks in the second row (as indicated in the parenthesis as“client”). The sequence of frame transmission advances in time from leftto right, as represented by the arrow time line 401. For instance, forthe exemplary communication shown in FIG. 4, an RTS frame sent by the AP202 begins the sequence, and a CF-end sent by the AP 202 ends thesequence. Note that the first address (e.g., BC) is the receiveraddress, which should be the broadcast address so that the CF-end isreceived by all stations. The second address (e.g., BSSID) is usuallythe transmitter address, but in one or more embodiments describedherein, comprises the unicast destination address.

In particular, the AP 202 (not shown, but implied as denoted by the “AP”in parenthesis) sends an RTS frame (or RTS) 402, and the client 204 (notshown, but implied as denoted by the “client” in parenthesis) respondsby sending a CTS frame (or CTS) 404. The sending of the RTS 402 and CTS404 results in the setting of a NAV around (e.g., close enough to decodeframe transmissions all or at least most of the time) the respectivedevices. The AP 202 sends one or more data frames 406, each of which areacknowledged by the client 204 with an acknowledgement (ACK) frame 408.The final data frame 406 from the AP 202 to the client 204 is “tagged”with an end of TXOP (EOT) indication. This EOT indication comprises asignal or flag to the client 204 to respond with a CF-end frame 410after a SIFS interval. In some implementations, the CF-end frame 410 maybe preceded by a mandatory response frame, such as the ACK frame 408 (ora block acknowledgement (BA) frame, the latter not shown). A basicservice set identifier (BSSID) field of the CF-end frame 410 carries aBSSID (e.g., MAC address) of the AP 202. The AP 202 recognizes its BSSIDin the received CF-end frame 410 and responds to this CF-end frame 410with a CF-end frame 412. By the AP 202 sending the CF-end frame 412, theTXOP is actively truncated by both devices 202 and 204.

FIG. 5 illustrates an alternative to the mechanism shown in FIG. 4. Thatis, the final frame in the TXOP sent by the AP 202 is a CF-end frame 502comprising the address (e.g., MAC address) of the client 204 inside theBSSID field. The client 204 recognizes its address in the CF-end frame502 and responds after a SIFS interval with a CF-end frame 504. TheCF-end frame 504 also comprises the client address inside the locationof the BSSID field. This mechanism illustrated in FIG. 5 is referred toas a “CF-end to self.”

FIG. 6 illustrates an alternative to the mechanism shown in FIG. 4. TheAP 202 sends a CF-end frame 602 with the client MAC address inside theBSSID field. The client responds with a CF-end frame 604 having theBSSID of the AP 202, to which the AP 202 responds with a CF-end frame606 with the BSSID of the AP 202. Hence, the AP 202 acts as the finaltransmitter of the final CF-end frame, which may be desirable inrelation to the EIFS inside a BSS since all associated clients receivethe final CF-end frame (e.g., CF-end frame 606) from the AP 202. Thefirst CF-end frame 602 uses a non-basic rate, and thereby functionssimilarly to the EOT frame indication (of final frame 406) shown in anddescribed in association with FIG. 4. By using a non-basic rate for thefirst CF-end frame 602, NAV truncation functionality is absent. That is,a CF-end frame is effective for NAV truncation when sent at a basic ratethat all clients inside a defined BSS can receive and decode.

Having described various exemplary mechanisms used by the STT system 200for providing symmetrical truncation, the following description (andassociated figures) generalizes some of the above-noted mechanisms forcommunications between stations (e.g., either AP 202 or clients 204,206) and provides additional mechanisms. That is, FIGS. 7-9 provide ageneralized illustration of the above-described mechanisms, in additionto describing additional mechanisms, by showing an exchange of framesbetween two stations, station 1 (STA1) (shown in a top row of FIGS. 7-9)and station 2 (STA2) (shown in a second row of FIGS. 7-9). In someembodiments, there may be an exchange of frames with a third stationsuch as an AP or other device (shown in a third row at the bottom ofFIGS. 8-9). The explanation of the various mechanisms shown in FIGS. 7-9is directed to the exchange of frames that commence after the exchangeof CTS/RTS frames and data and ACK frames, the latter of which havealready been described above in association with FIGS. 4-6.

FIG. 7 illustrates a general principle of certain embodiments of the STTsystem 200 that any station that receives a CF-end frame 702 with itscorresponding MAC address inside the location of the BSSID fieldresponds to the CF-end frame 702 with a CF-end frame 704. The addressused inside the CF-end frame 702 depends on the specific implementation.In general, if a CF-end frame 704 is required (a determination to bemade by a client or an AP), the address of the transmitter of the nextCF-end frame 704 is included inside the location of the BSSID field ofCF-end frame 702. If no CF-end frame 704 (also referred to herein as aCF-end response frame) is required and the CF-end frame 702 is the finalframe of the sequence, the station's (e.g., STA2) own address isincluded in the BSSID field, or the broadcast address (BC), or any otheraddress which is not immediately associated with another stationaddress. Such a scheme is applicable in an IBSS or Direct Linkenvironment as well. In general, a client in infrastructure mode wouldtypically include the BSSID of the AP, so that the AP generates a CF-endresponse. Further, a client in IBSS mode may include the MAC address ofthe peer node to which it is communicating during the TXOP. An AP may ormay not include an address other than a BSSID of the AP.

Consider the case where STA1 comprises a client station 204, and STA2comprises an AP station (or simply, AP) 202. The mechanism shown in FIG.7 enables the AP 202 to respond to a CF-end 702, which contains amatching BSSID (e.g., MAC address) with another CF-end 704 after SIFS(which in some implementations, may be a PIFS interval). The CF-end 702transmitted by the client station 204 comprises the final frame in theTXOP of client station 204, and in one embodiment, is an unacknowledgedframe (hence, it is safe to transmit a CF-end 704 after SIFS). The BSSIDmakes the CF-end 702 specific to one AP, avoiding collisions betweenmultiple APs transmitting a CF-end 704. The CF-end 704 clears the NAVaround the AP 202, which covers the entire BSS. Note that someimplementations may not require the CF-end 704. For instance, when thedistance between the AP 202 and the client station 204 is small, asingle CF-end from either the AP 202 or the client station 204 roughlyresets the EIFS in the same coverage area, because the coverage areasfor both devices largely overlap. The determination by the AP 202 tosend a CF-end response (CF-end 704) can be based on the estimateddistance of associated client stations 204, 206, and possibly incombination with the estimated distance of the transmitter of theCF-end. Such a determination can, for instance, be based on receivedsignal strengths, or the PHY rates or modulation code schemes (MCSs)used to communicate with the client stations 204, 206. Other mechanismsmay be used in the determination.

Continuing the case where STA1 is a client 204 and STA2 is an AP 202,the client 204 may send a CF-end 702 with a duration value (which sets aNAV) corresponding to a defined time after the expected CF-end 704 to besent by the AP 202. The AP 202 sends a CF-end 704 (CF-end response) witha duration of zero, which causes receivers of each CF-end to resumebackoff at exactly, or substantially exactly, the same time (and hence,in one embodiment, superceding existing rules that allow an EIFS tofollow a CF-end). The client 204 may also send a CF-end 702 with aphysical layer convergence protocol (PLCP) duration (which is signaledinside a SIG field or L-SIG field of the frame) equal to the CF-enditself plus SIFS plus the expected CF-end response duration from the AP202. The AP 202, in one embodiment, is allowed to ignore this physicalduration and respond with a CF-end 704 a SIFS interval after the end ofthe actual transmission of the CF-end 702, as can be determined from theindicated rate and size of the CF-end 702 (e.g., 20 octets without an HTcontrol field, 24 octets with an HT control field). The CF-end 704, inone embodiment, comprises a regular PLCP duration. The mechanismsdescribed below in association with FIGS. 14 and 15 address situationswhere there is a difference in when the NAV gets reset and backoffresumes.

In some embodiments, a third CF-end frame may be added to the framesequence illustrated in FIG. 7 when the second station includes theBSSID. Such a situation is illustrated in FIG. 8, which includes theaddition of an AP (implicitly represented by the bottom or third rowframe 802). The TXOP is initiated by STA1 sending a CF-end frame 802 toSTA2, which responds with a CF-end frame 804 to the AP. As shown, the APresponds with a final CF-end frame 806 with the BSSID inside the BSSIDfield of CF-end frame 806.

In an IBSS implementation, the BSSID may be recognized by the stationthat transmitted the final IBSS beacon, as illustrated in FIG. 9. Asshown, STA1 sends a CF-end frame 902 addressed at STA2. STA2 respondswith a CF-end frame 904 addressed at the BSSID of STA2. STA3 respondswith a CF-end frame 906. The BSSID can be the address of the station(e.g., STA3) that transmitted the latest beacon in an IBSS, or it can bethe address of the AP in case the communication between STA1 and STA2comprises a Direct Link communication.

In some embodiments, the receiver address (RA) of a CF-end frame isequal to the broadcast address, which ensures that all stations withinrange receive the CF-end frame and process the same. The location of theBSSID can be used to indicate the destination of the CF-end. Further, insome embodiments, the recognition of the CF-end type/subtype is enoughfor a receiver to truncate the NAV. In such implementations, the RA ofthe CF-end frame can be used to store the CF-end responder address,while the BSSID field can be used to store the BSSID.

Having described various embodiments of the STT system 200 above, it canbe appreciated in the context of this disclosure that one methodembodiment, referred to as STT method 200 a corresponding to themechanism shown in FIG. 4 and shown in the flow diagram of FIG. 10,comprises sending by an AP a final data frame of a transmit opportunity(TXOP) with an end of transmit opportunity (EOT) indicator includedtherein (1002), receiving by a client the final data frame (1004), theclient responding by sending a CF-end frame, with an identifier (e.g.,MAC address, etc.) of the AP included therein, to the AP to truncate theTXOP around the client (1006), the AP receiving the CF-end frame withthe identifier (1008), and the AP sending a CF-end frame with the sameidentifier to the client to truncate the TXOP around the AP (1010).

It can be appreciated that portions of the method embodiment 200 a areimplemented at each device, and hence such portions can be describedfrom the perspective of the individual devices of the system 200. Forinstance, one method embodiment corresponding to method 200 a, viewedfrom the perspective of an AP, comprises sending (by an AP) a final dataframe of a transmit opportunity (TXOP) with an end of transmitopportunity (EOT) indicator included therein, and responsive toreceiving a CF-end frame with an identifier of the AP included thereinto truncate the TXOP around the client, sending a CF-end frame with thesame identifier to the client to truncate the TXOP around the AP.

Another method embodiment corresponding to method 200 a, from theperspective of a client, comprises receiving (by a client) a final frameof a transmit opportunity (TXOP) from an AP, the final data frame havingan end of transmit opportunity (EOT) indicator included therein,responding to the final frame by sending a CF-end frame, with anidentifier of the AP included therein, to the AP to truncate the TXOParound the client and to prompt the AP to send a CF-end frame totruncate the TXOP around the AP.

Another embodiment, referred to as STT method 200 b corresponding to themechanisms illustrated in FIGS. 5 and 7 and shown in the flow diagram ofFIG. 11, comprises sending, by a first station (e.g., an AP), at the endof a TXOP a CF-end frame with an address of a second station (e.g., aclient) included therein to truncate a TXOP around the first station(1102), receiving by the second station the CF-end frame from the firststation (1104), the second station responding to the first station bysending a CF-end frame with the address of the second station includedtherein to truncate the TXOP around the second station (1106).

It can be appreciated that portions of the method embodiment 200 b areimplemented at each device (e.g., stations, whether configured as an APor client), and hence such portions can be described from theperspective of the individual devices of the system 200. For instance,one method embodiment corresponding to method 200 b implemented at afirst station (e.g., AP) comprises receiving a final frame of a TXOPfrom a second station (e.g., client) at the first station, andresponsive to receiving the final frame of the TXOP, sending, at the endof the TXOP, a CF-end frame with an address of the second stationincluded therein to truncate a TXOP around the first station and toprompt the second station to send a CF-end frame to truncate the TXOParound the second station.

Additionally, one method embodiment corresponding to method 200 bimplemented at a second station (e.g., client) comprises receiving (atthe second station) a CF-end frame with an address of the second stationincluded therein, the CF-end used to truncate a TXOP around a firststation (e.g., AP), and responding to the first station by sending aCF-end frame with the address of the second station included therein totruncate the TXOP around the second station.

Another method embodiment, referred to as STT method 200 c correspondingto the mechanism illustrated in FIG. 6 and shown in the flow diagram ofFIG. 12, comprises an AP sending a CF-end frame at a non-basic rate (andhence acting as an EOT indicator) with the address of the clientincluded therein (1202), the client receiving the CF-end frame (1204),the client responding by sending a CF-end frame having the address ofthe AP included therein, which truncates the TXOP around the client(1206), the AP receiving the CF-end frame sent by the client (1208), andthe AP responding by sending a CF-end frame with the address of the APincluded therein, resulting in the AP acting as the final transmitter ofthe final CF-end frame that truncates the TXOP around the AP (1210).

It can be appreciated that portions of the method embodiment 200 c areimplemented at each device, and hence method embodiments can bedescribed from the perspective of the AP and client. One methodembodiment corresponding to method 200 c implemented at an AP comprisesan AP sending a CF-end frame at a non-basic rate (and hence acting as anEOT indicator) with the address of the client included therein, andresponsive to receiving a client-TXOP truncating CF-end frame having theaddress of the AP included therein, responding by sending a CF-end framewith the address of the AP included therein, which truncates the TXOParound the AP.

Additionally, one method embodiment corresponding to method 200 cimplemented at a client comprises receiving by a client a CF-end framepossibly at a non-basic rate (and hence acting as an EOT indicator) withthe address of the client included therein, and responding by sending aCF-end frame having the address of the sending AP included therein,which truncates the TXOP around the client and prompts the AP to send afinal CF-end frame that truncates the TXOP around the AP.

Another method embodiment, referred to as STT method 200 d correspondingto the mechanisms illustrated in FIGS. 8-9 and shown in the flow diagramof FIG. 13, comprises sending, by a first station, at the end of a TXOPa CF-end frame with an address of a second station included therein totruncate a TXOP around the first station (1302), receiving by the secondstation the CF-end frame from the first station (1304), the secondstation responding to the first station by sending a CF-end frame withan identifier included therein to a third station to truncate the TXOParound the second station (1306), the third station receiving the CF-endresponse frame (1308), the third station responding by sending a CF-endresponse frame with the identifier therein to the second station totruncate the TXOP around the third station (1310).

It can be appreciated by one having ordinary skill in the art, in thecontext of this disclosure, that each device participating in themethodology illustrated in FIG. 13 performs portions of the describedmethod 200 d that can be described similarly to the manner suchperspective-based methods have been described previously in FIGS. 10-12,and thus omitted here for brevity.

FIG. 14 illustrates various mechanisms for client-initiated TXOPs forembodiments of the STT system 200, and in particular, mechanisms thatfocus on issues of NAV reset and backoff resumption involving CF-endframes or frames acting in particular circumstances with similarfunctionality to CF-end frames. The description that follows is based inpart on the previous discussion corresponding to FIG. 7 for the casewhere STA1 is a client and STA2 is an AP. To solve the difference inwhen a NAV gets reset and the backoff resumes, the client may transmit ashort frame (EIFS set frame) 1404 with a defined frame check sequence(FCS) field 110 (FIG. 1), which is not the correct FCS (e.g., if the FCSis correct, then stations start an EIFS rather than a DCF InterframeSpace/Arbitration Interframe Space or DIFS/AIFS), at a SIFS time afterthe CF-end frame 1402. The transmission of this short frame 1404 in themanner as described above causes receivers (e.g., clients 204, 206) tostart an EIFS. The AP 202 delays (e.g., by SIFS+EIFS setframe+EIFS-DIFS-CF-end frame, represented by duration interval 1401) thesending of the CF-end frame 1406 so that the transmission of frame 1406finishes exactly (or substantially so) when the EIFS-DIFS finishes (theEIFS-DIFS represented by the duration interval denoted by 1403), so thateither client (e.g., 204, 206) starts DIFS (or more general, AIFS) atthe same time. The DIFS refers to a first fixed part of the DCF backoff(DIFS), and the AIFS refers to the EDCA backoff (AIFS). As is known,EDCA is similar to DCF but with QoS differentiation (802.11e). The AP202 does not start an EIFS and ignores the intermediate EIFS set frame1404. Thus, the EIFS set frame 1404 causes AIFS to start for clients204, 206 that cannot receive the CF-end 1406 from the AP 202. The CF-endframe 1406 from the AP 102 is transmitted during this time, such thatall nodes start DIFS (or AIFS) at the same time. Note that in someembodiments, the EIFS set frame 1404 can be a zero length frame (e.g., apreamble without a MAC payload).

FIG. 15 is a block diagram that illustrates the principles of themechanism shown in FIG. 14, without the use of CF-end frames. In otherwords, the CF-end frame 1402 of FIG. 14 is replaced with an EOT frame1502, and the CF-end frame 1406 of FIG. 14 is replaced with an ACK frame1504. The mechanism illustrated in FIG. 15 can be used to resume thebackoff for all nodes within the range of either the client 204 or theAP 202 or both in the network at the same time (e.g., without part ofthe nodes having to wait for an EIFS to finish). The EOT frame 1502 issent to the AP 202, followed after an SIFS interval by the EIFS setframe 1404. The AP 202, through a similar delay 1401 and similar mannerto the mechanism described in association with FIG. 14, responds to theEOT frame 1502 (and ignores the EIFS set frame 1404) with the ACK frame1504. The ACK frame 1504 ends at the same time as the interval 1403after the end of the EIFS set frame 1404. Note that in some embodiments,both the AP and the client can start an EIFS set frame at the same, orsubstantially same, time.

Having described the various mechanisms shown in FIGS. 14-15, one methodembodiment of the STT system 200 encompassing these mechanisms, themethod referred to as 200 e and shown in FIG. 16, comprises the client204 sending a final data frame of a TXOP to the AP 202 (1602), the finalframe having the form of a CF-end frame specific to a given AP or an EOTframe, sending an EIFS set frame (1604), receiving by the AP 202 theCF-end frame after a defined interval (the defined interval being aSIFS, a PIFS, or duration 1401 described in association with FIG. 14)(1606), and responsive to deciding to send the CF-end response frame orACK frame, clearing the NAV for the network (e.g., BSS) by sending theCF-end response frame or ACK frame to the client 204 (1610). Responsiveto deciding not to send the CF-end response frame, the CF-end frame actsto clear the NAV (1612), as described above.

It can be appreciated by one having ordinary skill in the art, in thecontext of this disclosure, that each device participating in themethodology illustrated in FIG. 16 performs portions of the describedmethod 200 e that can be described similarly to the manner suchperspective-based methods have been described previously in FIGS. 10-12,and thus omitted here for brevity.

Any process descriptions or blocks in flow charts should be understoodas representing modules, segments, or portions of code which include oneor more executable instructions for implementing specific logicalfunctions or steps in the process, and alternate implementations areincluded within the scope of the present disclosure in which functionsmay be executed out of order from that shown or discussed, includingsubstantially concurrently or in reverse order, depending on thefunctionality involved, as should be understood by those reasonablyskilled in the art of the present disclosure.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations,merely set forth for a clear understanding of the principles of thedisclosure. Many variations and modifications may be made to theabove-described embodiment(s) of the disclosure without departingsubstantially from the spirit and principles of the disclosure. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure and the present disclosure and protected bythe following claims.

What is claimed is:
 1. A symmetric transmit opportunity (TXOP)truncation method comprising: receiving a frame from an access point(AP) that includes an indicator of an end of a TXOP; and responsive toreceiving the indicator, sending to the AP a first frame that truncatesthe TXOP around a first station, wherein the AP responds to receipt ofthe first frame by sending a second frame that truncates the TXOP aroundthe AP.
 2. The method of claim 1, wherein receiving the frame comprisesreceiving a data frame.
 3. The method of claim 1, further comprisingsending an acknowledgement (ACK) frame before sending the first frame tothe AP.
 4. The method of claim 1, wherein sending the first frame andsending the second frame comprises sending the first frame and thesecond frame with an address of the AP.
 5. The method of claim 4,wherein the address of the AP comprises an address in the BSSID field ofthe first or second frame.
 6. The method of claim 1, wherein the firstframe is sent at a non-basic rate, and the second frame is sent at abasic rate.
 7. A client device comprising: a client receiver configuredto receive a frame from an access point (AP) that includes an indicatorof an end of a TXOP; and a client transmitter configured to, in responseto receiving the indicator, send a first frame that truncates the TXOParound the client device, wherein the AP responds to receipt of thefirst frame by sending a second frame that truncates the TXOP around theAP.
 8. The client device of claim 7, wherein the client receiver isconfigured to receive a data frame.
 9. The client device of claim 7,wherein the client receiver is further configured to send anacknowledgement (ACK) frame before sending the first frame to the AP.10. The client device of claim 7, wherein the client transmitter isconfigured to send the first frame and the second frame with an addressof the AP.
 11. The client device of claim 10, wherein the address of theAP comprises an address in the BSSID field of the first or second frame.12. The client device of claim 7, wherein the first frame is sent at anon-basic rate, and the second frame is sent at a basic rate.
 13. Theclient device of claim 7, wherein the first frame is a CF-end frame. 14.A non-transitory, computer-readable medium having instructions storedthere-on for performing a symmetric transmit opportunity (TXOP)truncation between a first station and a second station, theinstructions comprising: instructions for receiving a frame from anaccess point (AP) that includes an indicator of an end of a TXOP; andinstructions for, responsive to receiving the indicator, sending to theAP a first frame that truncates the TXOP around a first station, whereinthe AP responds to receipt of the first frame by sending a second framethat truncates the TXOP around the AP.
 15. The computer-readable mediumof claim 14, wherein receiving the frame comprises receiving a dataframe.
 16. The computer-readable medium of claim 14, further comprisingsending an acknowledgement (ACK) frame before sending the first frame tothe AP.
 17. The computer-readable medium of claim 14, wherein sendingthe first frame and sending the second frame comprises sending the firstframe and the second frame with an address of the AP.
 18. Thecomputer-readable medium of claim 17, wherein the address of the APcomprises an address in the BSSID field of the first or second frame.19. The computer-readable medium of claim 14, wherein the first frame issent at a non-basic rate, and the second frame is sent at a basic rate.20. The computer-readable medium of claim 14, wherein the first frame isa CF-end frame.